A furnace, such as a plasma arc or electron beam cold hearth melting furnace, for example, can melt and cast material for periods of time. During continuous casting operations, molten material can continuously enter a mold and cast material, or ingot, can continuously emerge from the mold. For example, molten material can flow into the top of the mold while a withdrawal mechanism continuously translates to allow cast material to emerge from the bottom of the mold. Continuous casting can reduce the frequency of interruptions to casting operations, such as delays associated with changing the mold between casting cycles, for example. Reducing interruptions during casting operations can increase casting efficiency.
Some materials are reactive when molten or at high temperature. A material that is reactive in this way, when in a molten state or heated to or above a particular temperature, will readily chemically combine or otherwise chemically change when exposed to certain elements or compounds. For example, molten titanium and solid cast titanium at very high temperature are reactive and readily chemically combine with gaseous oxygen to form titanium dioxide and with gaseous nitrogen to form titanium nitride. Titanium dioxide and titanium nitride may form hard alpha defects in cast titanium and make it unsuitable for intended applications. Consequently, molten titanium and high temperature cast titanium preferably are maintained in a vacuum or in an inert atmosphere during certain stages of the casting operation. In an electron beam cold hearth furnace, a high or substantial vacuum is maintained in the melting and casting chambers to allow the electron beam guns to operate. In a plasma arc cold hearth furnace, plasma torches use an inert gas such as helium or argon, for example, to produce plasma. Accordingly, in a plasma arc cold hearth furnace, the presence of the inert gas for the plasma torches generates a pressure in the furnace that can range from sub-atmospheric to a positive pressure. If the melt chamber of a plasma arc or electron beam cold hearth melting furnace is infiltrated with a non-inert gas, such as oxygen or nitrogen, for example, the non-inert gas can contaminate the molten material therein. Thus, gas from the external atmosphere should be completely or substantially prevented from entering the melt chamber of a furnace containing molten titanium.
It would be advantageous to provide a continuous casting system that is less susceptible to contamination of titanium or another reactive material contained therein. More generally, it would be advantageous to provide an improved continuous casting system that is useful for titanium, other reactive materials, and metals and metal alloys generally.